There are many forms of exercise devices on the market today and a variety of exercise devices are the subjects of prior patents. One common type of exercise device is an exercise bicycle sometimes called a "bicycle ergometer". In this type of device, the user expends energy by pedaling against a resistance or braking element forming part of the bicycle. For example, in a mechanical exercise bicycle, the resistance to pedaling experienced by the user is provided by a friction brake which normally includes friction pads that act against a rotating disk. The clamping pressure of the friction pads is usually adjustable to vary the load against which the user pedals.
In other type of exercise bicycles, the load is electrically generated. For example, some currently available exercise bicycles utilize an "eddy-current brake". In this type of bicycle, a disk, mechanically coupled to a pedaling mechanism, is rotated at high speed. Radial surfaces of the disk pass between an electromagnet pole assembly which when energized applies a braking force against the disk. The braking force or load felt by the user is adjustable and is a function of the energization level of the electromagnetic assembly. With this type of loading device, an outside source of power must be connected to the bicycle in order to energize the electromagnetic poles of the eddy-current brake. In addition, the "eddy-current" phenomena relies on the precise positioning of the disk with respect to the magnet pole assembly. As a result, it is believed that the eddy-current brake is relatively costly to manufacture and maintain. Another disadvantage of this type of exercise device is that a direct measurement of the load or work output of the eddy-current brake is not possible. If information regarding load or even operating speed is desired, a separate load or speed sensor must be used. An example of an exercise bicycle that utilizes an eddy-current brake is described in U.S. Pat. No. 4,678,182.
Exercise bicycles utilizing AC or DC generators as load devices have also been suggested. For example, U.S. Pat. No. 3,765,245 illustrates an ergometer which includes a patient driven, three phase AC generator as the loading device. The energy generated upon rotation of the armature by the patient via a pedaling mechanism, is dissipated across a controllable variable resistance load which comprises a load resistor and a power transistor connected in a series.
U.S. Patent No. 3,057,201 illustrates an example of an exercise bicycle that uses a DC generator as a load device. In the disclosed ergometer, a control circuit regulates the excitation of the field winding of the DC generator in order to control its power output independent of the operating speed of the ergometer. With the disclosed arrangement, an outside source of power is needed. The control circuit controls the application of the external power to the field windings of the generator in order to change the field excitation and hence, the load exerted on the user.
Commercially available exercise bicycles, designed for general fitness applications, often provide a workload that is a function of pedaling speed. When these bicycles are used for cardiac rehabilitation, the patient is instructed to pedal at a prescribed speed and maintain that speed constant during the exercise time. It has been found that due to either lack of concentration, perceived discomfort or distractions, the patient will usually pedal at speeds other than prescribed and thus the workload exerted by the patent will vary accordingly.
In general fitness applications, this does not normally pose a problem. However, in cardiac rehabilitation, it is important that the recovering patient operate the exercise device at a constant workload. For this reason, exercise bicycles in which the workload varies with pedaling speed, are not considered acceptable for cardiac rehabilitation applications.
For cardiac rehabilitation applications, it is also important that the exercise device maintain a relatively constant calibration from one exercise session to another so that performance changes by a given patient can be closely monitored over the entire rehabilitation process. In other words, it is important that the physician be able to set the same power level on the bicycle for each session it is used by a given patient.